644 Mr. Nikola Tesla [Feb. 4, 



speed, in synchronism with the alternations, say 10,000 times a second. 

 The rotation can be slowed down or accelerated by the approach or 

 receding of the observer, or any conducting body, but it cannot be 

 reversed by putting the bulb in any position. Very curious experi- 

 ments may be performed with the brush when in its most sensitive 

 state. For instance, the brush resting in one position, the experi- 

 menter may, by selecting a proper position, approach the hand at a 

 certain considerable distance to the bulb, and he may cause the brush 

 to pass oif by merely stiffening the muscles of the arm, the mere 

 change of configuration of the arm and imperceptible displacement 

 being sufficient to disturb the delicate balance. When it begins to 

 rotate slowly, and the hands are held at a proper distance, it is 

 impossible to make even the slighest motion without producing a 

 visible effect upon the brush. A metal plate connected to the other 

 terminal of the coil affects it at a great distance, slowing down the 

 rotation often to one turn a second. 



It is hoped that this phenomenon will prove a valuable aid in the 

 investigation of the nature of the forces acting in an electrostatic or 

 magnetic field. If there is any motion which is measurable going on 

 in the space, such a brush would be apt to reveal it. It is, so to 

 speak, a beam of light, frictionless, devoid of inertia. 



On account of its marvellous sensitiveness to electrostatic or 

 magnetic disturbances it may be the means of sending signals through 

 submarine cables with any speed, and even of transmitting intelli- 

 gence at distance without wires. 



In operating an induction coil with these rapidly alternating 

 currents, it is astonishing to note, for the first time, the great import- 

 ance of the relation of capacity, self-induction, and frequency as 

 regards the general result. The combination of these elements pro- 

 duces many curious effects. For instance, two metal plates are con- 

 nected to the terminals and set at a small distance, so that an arc is 

 formed between them. This arc prevents a strong current to flow 

 through the coil. If the arc be interrupted by the interposition of 

 a glass plate, the capacity of the condenser obtained counteracts the 

 self-induction, and a stronger current is made to pass. The effects 

 of capacity are the most striking, for in these experiments, since the 

 self-induction and frequency both are high, the critical capacity is 

 very small, and need be but slightly varied to produce a very con- 

 siderable change. The experimenter brings his body in contact with 

 the terminals of the secondary of the coil, or attaches to one or both 

 terminals insulated bodies of very small bulk, such as exhausted 

 bulbs, and he produces a considerable rise or fall of potential on the 

 secondary, and greatly affects the flow of the current through the 

 primary coil. 



In many of the phenomena observed, the presence of the air, or; 

 generally speaking, of a medium of a gaseous nature (using this term 

 not to imply specific properties, but as contradistinction to homo- 

 geneity or perfect continuity) plays an important part, as it allows 



